This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/EP99/09401 which has an International filing date of Dec. 1, 1999, which designated the United States of America.
1. Field of the Invention
The invention relates to a combustion device for the combustion of a fuel, the fuel being capable of being supplied to the combustion as a fluid stream via a supply duct. The invention also relates to a corresponding method.
2. Background of the Invention
In the book xe2x80x9cTechnische Strxc3x6mungslehrexe2x80x9d [xe2x80x9cTechnical Fluid Mechanicsxe2x80x9d] by Willi Bohl; 10th edition, Vogel-Verlag, Wurzburg 1994, outflow processes are described in Chapter 5.6. Illustrated in more detail are processes for the outflow of a fluid from a vessel, in which the fluid is stored at the pressure pi and the density xcfx81i. The fluid emerges from the vessel as a jet, the jet pressure Pa prevailing in the jet. The pressure ratio in the case of which, in the event of a given vessel state, that is to say a given vessel pressure pi and a given fluid density xcfx81i and also in the case of a given vessel orifice from which the fluid emerges, the mass flow of fluid no longer changes, is designated as the critical pressure ratio (Pa/Pi)(k). Depending on the size of the pressure ratio Pa/Pi, a distinction is made between two types of outflow processes: 1. Subcritical outflow; 2. Supercritical outflow.
A Laval nozzle is described in section 5.6.2 of the same book. The Laval nozzle serves for expanding the outflowing fluid beyond the critical pressure ratio and for consequently increasing the flow velocity beyond sound velocity. For this purpose, the fluid is first compressed by a narrowing duct, the flow velocity increasing up to sound velocity. This is followed by a widening duct section, in which the fluid expands and the flow velocity reaches the supersonic range. Such a Laval nozzle serves, for example, for achieving maximum outflow velocities for thrust gases of rocket propulsion units. FIG. 5.25 illustrates various operating states of a Laval nozzle. In the operating state illustrated first, the outlet pressure of the fluid is above the critical pressure. The Laval nozzle behaves, here, in the same way as a Venturi tube. More detailed particulars as to the definition of a Venturi tube follow further below.
Compression flows are described in section 5.7 of the same book. Section 5.7.1 explains the functioning of a subsonic diffuser. Subsonic diffusers are ducts which are widened in the direction of flow and in which a flow running in the subsonic range is decelerated. The deceleration results in a pressure rise. Subsonic diffusers are found, for example, in jet appliances, Venturi tubes and in the guide wheels and outlet casings of turbocompressors. Section 5.7.2 describes a supersonic diffuser, in which the duct cross section narrows in the direction of flow.
European Standard EN ESO 5167-1 relates to throughflow measurements of fluids by means of throttle devices. Diaphragms, nozzles and Venturi tubes in full-throughflow lines of circular cross section are described in Part 1. FIG. 10 shows a classic Venturi tube. A fluid flows through the Venturi tube in a direction of flow. The Venturi tube consists of an entry cone narrowing in the direction of flow and of a widening exit cone following the entry cone in the direction of flow. A pronounced pressure loss occurs in the entry cone. This pressure loss is for the most part compensated again by the exit cone, so that the overall pressure loss occurring through the Venturi tube, as compared with a tube of invariable cross section and the same length, remains low.
In the book xe2x80x9cBerechnung der Schallausbreitung in durchstrxc3x6mten Kanxc3xa4len von Turbomaschinen unter besonderer Berxc3xccksichtigung der Auslegung von Drehtonschalldxc3xa4mpfernxe2x80x9d [xe2x80x9cCalculation of the sound propagation in throughflow ducts of turbomachines, taking particular account of the design of rotational sound dampers xe2x80x9d] by Christian Faber, Verlag Shaker, Aachen 1993, section 3.4 illustrates how discontinuities in flow ducts influence the propagation of sound in a fluid flowing in these flow ducts. Scatter, reflection and transmission factors are derived, from which it is possible to calculate which part of incident sound energy passes the discontinuity and which part is reflected.
One object of the invention is to specify a combustion device which has beneficial properties in terms of controlling and influencing the propagation and formation of sound waves induced by combustion. A further object of the invention is to specify a corresponding method.
These and other objects are achieved, according to the invention, by specifying a combustion device for the combustion of fuel, with a supply duct for supplying the fuel to a combustion zone, the fuel being capable of being guided through the supply duct as a fluid stream with a direction of flow and at a nominal velocity lying within a nominal operating interval, and the supply duct being narrowed in a decoupling region, in such a way that, at the nominal velocity, sound waves running opposite to the direction of flow in the fluid stream from the combustion zone are at least partially reflected in the decoupling region.
During combustion, combustion oscillations may arise, in that, in the event of a fluctuation in a release of power during combustion, a pressure pulse occurs in the fluid stream. Such a pressure pulse in the fluid stream results, in turn, in unevenness in the mass flow of the fluid stream entering the combustion zone. This again leads to a release of power fluctuating in time during combustion. Depending, for example, on the geometric designs of the supply duct, positive feedback may be produced between pressure pulses in the fluid stream and the fluctuating release of power during combustion. A combustion oscillation is formed. Such a combustion oscillation may have a disturbing effect, for example, in the form of considerable noise pollution. In the case of large releases of power, however, vibrations may also occur in the combustion device, which may ultimately result in damage. The invention proceeds from the knowledge that the propagation of sound waves in the fuel via the supply duct into further acoustically coupled regions is conducive to the tendency to the formation of such combustion oscillations. This mechanism is prevented by an acoustic decoupling of the supply duct or else of a plurality of supply ducts for the fuel. Such acoustic decoupling is achieved by a narrowing of the supply duct or supply ducts.
By virtue of such narrowing in the direction of flow, known hitherto only with regard to airborne sound dampers, the flow velocity of the fluid is increased. In this case, the flow velocity may be increased to an extent such that sound waves running toward the narrowing opposite to the direction of flow are reflected. The narrowing is designed in such a way that, at a nominal velocity of the fluid stream in the supply duct, such a high acceleration of the fluid is obtained at the narrowing that a high proportion of the sound waves running toward the narrowing is reflected. The nominal velocity is, for example, within a velocity interval which corresponds to those operating states of the combustion device in which there is a high tendency to the formation of combustion oscillations.
The uncoupling region is designed preferably as a continuous narrowing of the supply duct in the direction of flow. At such a continuous narrowing, lower flow and pressure losses due to turbulence are obtained, as compared with a discontinuous narrowing. Such a continuous narrowing could, for example, be designed in a similar way to the supersonic diffuser described by Willi Bohl in the above-mentioned book.
Preferably, the decoupling region is followed in the direction of flow by a pressure increase region which corresponds to a widening of the supply duct. The pressure in the fluid stream is increased by means of such a pressure increase region. This is carried out as a result of the widening of the supply duct. The passage consisting of the decoupling region and of the pressure increase region thus corresponds, for example, to the Venturi tube illustrated in the European standard specified above or to a Laval nozzle. Such an embodiment is advantageous, in particular, when a high fluid mass flow has to be provided. Thus, what is achieved by the combination of the uncoupling region and pressure increase region is that in the combustion device, a large release of power can be obtained with the aid of a high fluid mass flow, while at the same time an effective acoustic decoupling of the combustion zone and supply duct is provided.
The combustion zone is preferably located in a combustion chamber. The combustion chamber may have any desired shape, but it is particularly important to have a tubular or annular combustion chamber. In a combustion chamber, combustion oscillations may be formed as a result of interaction between a power fluctuation during combustion and characteristic acoustic modes of the combustion chamber. Such combustion chamber oscillations may be propagated into fluidically coupled spaces, for example advancing into the fuel or air supply lines and, in some circumstances as far as a supply pump, which may thereby be subjected to high mechanical load. Acoustic decoupling by means of the narrowing of the supply duct prevents such a propagation of the combustion chamber oscillations. Moreover, the tendency to the formation of combustion chamber oscillations at all is reduced, since the acoustic resonance space available for the combustion chamber oscillations is reduced as a result of the decoupling of the supply duct.
The combustion device is preferably a gas turbine, in particular with an annular combustion chamber. In a gas turbine, there is a particularly high release of power during combustion. Combustion oscillations may therefore lead, here, to particularly serious noise pollution and damaging vibrations. In an annular combustion chamber, it is virtually impossible to predict characteristic acoustic modes due to the complicated geometry, so that the formation of combustion chamber oscillations is particularly difficult to prevent here. Acoustic decoupling between the annular combustion chamber and the supply ducts for the combustion media is therefore of particular importance here.
Objects are further achieved, according to the invention, by specifying a method for the combustion of fuel, the fuel being supplied to a combustion zone as a fluid stream with a direction of flow and at a nominal velocity lying within a nominal operating interval, and the fluid stream being narrowed in a decoupling region, in such a way that, at the nominal velocity, sound waves running opposite to the direction of flow in the fluid stream from the combustion zone are at least partially reflected in the decoupling region.
The advantages of such a method emerge correspondingly from the above statements regarding the advantages of the combustion device.
The fluid stream is preferably narrowed continuously in the direction of flow. Preferably, the pressure in the fluid stream is increased by a fluid stream widening which follows the narrowing. Also preferably, the fuel used is natural gas or oil. The fuel is preferably burnt in a combustion chamber of a gas turbine, in particular an annular combustion chamber.